In the art of window assemblies having outer main window frames surrounding sash frames and formed of extrusions of a rigid plastics material or polyvinylchloride, commonly referred to as vinyl or PVC, it has been found desirable in some installations, especially in residential and commercial buildings near the ocean and Gulf coast, to provide for keeping the window assembly in tact during a high wind load, for example, during a hurricane or tornado. For example, U.S. Pat. No. 6,826,871, which issued to the assignee of the present invention, discloses a double hung or single hung window unit constructed of vinyl extrusions and incorporating hook members on the sill and bottom sash frame member. The hook members engage in response to a high negative wind load to limit deflection of the sash frame and thereby avoid permanent deformation and/or separation of the sash frame from the main frame. It is also desirable to limit deflection of the sash frames or separation of the sash frame from the surrounding main frame when the sash frames and glass panel units are subjected to either a high negative or a high positive wind load and such a wind load after an impact by an object sufficient to break one or both of the glass panel units, such as, during a hurricane or tornado.